Electric discharge lamps having hot cathode



United States Patent Office 3,522,466 Patented Aug. 4, 1970 3,522,466 ELECTRIC DISCHARGE LAMPS HAVING HOT CATHODE Takeo Kamegaya, 365 Kyodo-cho, Setagaya-ku, Tokyo,

Japan; Naoyuki Takamura, 28 l-chome, Harumi-cho, Fuchu-shi, Tokyo, Japan; Mitsuo Inoue, 930 Koadach, Romae-cho, Kitatama-gun, Tokyo, Japan; Akira Someya, 176 4-chon1e, N oge-machi, Naka-ku, Yokohamashi, Japan; and Tsunekazu Hashimoto, 430 Seijo-cho, Setagaya-ku, Tokyo, Japan Continuation-impart of application Ser. No. 459,842,

May 28, 1965. This application July 29, 1968, Ser.

Int. Cl. H013 17/06, 61/08 US. Cl. 313212 2 Claims ABSTRACT OF THE DISCLOSURE In a positive column type low pressure vapor discharge lamp, the improvements wherein at least one self-sustained hot cathode electrode has an elongated electron emissive body or bodies having a distance around the surfaces of said body or bodies longer than 1.l /J mm., and providing an effective diameter less than l.2 J mm., where J represents the lamp current over the magnitude of from 0.2 ampere to 1.0 ampere.

This is a continuation-in-part of application Ser. No. 459,842, filed May 28, 1965, and now abandoned.

This invention relates to improvements relating to positive column type low pressure vapor discharge lamps provided with hot cathodes of the pre-heated or self-heating type and more particularly to improvements relating to cathode structures thereof.

While in the category of positive column type low ressure vapor discharge lamps are included various lamps such as fluorescent lamps, sterilizing lamps, black light lamps and the like, as illustrative examples thereof a fluorescent lamp will be considered. It is well known fact that when a fluorescent lamp is operated from a source of supply, particularly a source of alternating current a high frequency noise is created which acts on wireless receiving apparatus. This becomes a serious problem to be solved at once as a result of the wide spread use of fluorescent lamps. However most of the approaches which have been proposed to solve this problem relate to circuits outside the fluorescent tubes and do not provide any positive means to eliminate the source of noise or to minimize the noise. For example, use of noise preventing capacitors and the like merely decreases the noise but is not always satisfactory. Thus no effective means has been proposed to construct the fluorescent tube itself so that its creation of noise can be eliminated or minimized.

It is therefore the principal object of this invention to provide an improved construction of a hot cathode electrode for positive column type low pressure vapor discharge lamps, which can eliminate or minimize generation of noise which interferes with the operation of high frequency wireless receivers.

A further object of this invention is to provide positive column type low pressure vapor discharge lamps of long operating life wherein excessive concentration of cathode spot can be avoided without appreciably lowering or modifying the efliciency of the lamp as a source of light or its electrical characteristics when the condition of lighting varies.

Still another object of this invention is to provide positive column type low pressure vapor discharge lamps which can be constructed readily without substantially changing the construction of the envelope and electrode structure from what is known.

The invention can be more fully understood from the following detailed description taken in connection with the accompanying drawings in which:

FIG. la shows an enlarged side elevation of a cathode electrode constructed in accordance with this invention;

FIG. 1b is a cross-sectional view taken along the line II of FIG. 1a of an enlongated electron emissive body;

FIG. 2 is a graph to explain the relation between the separation of adjacent surfaces of an elongated body for electron emission and noise voltage;

FIG. 3 is a graph to explain the relation between the separation of adjacent surfaces of an elongated body for electron emission and the tube current for various fluorescent lamps for a constant noise voltage;

FIG. 4a shows a side view of a cathode electrode for discharge lamps embodying this invention;

FIG. 4b shows a plan view of the cathode electrode shown in FIG. 4a; and

FIGS. 5, 6 and 7 are plan views of modified cathode electrodes.

In order to obtain better understanding of this invention the relation between the discharge phenomenon at the ,time of starting a discharge lamp of the type referred to above and the state of thermal electron emission of the cathode electrode will be first considered to make clear the reasons for the generation of noise.

Florescent lamps of the rated capacity of from 10 to 40 watts which are now widely used in houses and oflices are generally provided with cathode electrodes made of tungsten wires in the form of double coils or triple coils coated with oxides for electron emisison.

Usually the length of the elongated cathode body coated with the oxide coating for electron emission is many times longer than the diameter thereof and the length and diameter are determined by considering such factors as preservation of heat for self-heating the cathode electrode, ability to support the required amount of electron emissive material, creation of local discharge at the time of starting a discharge lamp or the like whereas suitable voltages of the pitch and internal diameter of the coil forming the cathode heater are determined by considering a predetermined physical size of the cathode electrode.

In this case, thermal electrons emitted from a cathode spot formed at any one point on the surface of the cathode during operation of the fluorescent lamp are accelerated in the region of cathode drop in front of the cathode electrode so as to ionize a gas or mercury vapor contained in the lamp. In this ionizing region strong excitation is also effected to exhibit cathode glow. A portion of the ions created in this region diffuses widely in the space surrounding the region of cathode glow, thus disappearing by being recombined with electrons whereas the remaining ions are forced to flow towards the cathode electrode under the influence of an electric field. Particularly, at a temperature condition wherein the discharge current exceeds thermal electrons emitted from the cathode the electric field in front of the cathode increases sharply thus causing ions to be accelerated thereby to severely impinge upon the surface of the cathode electrode. As a result of increasing the temperature of the cathode spot the emission of thermal electrons is balanced with the magnitude of the discharge current. While thermal electrons emitted from the high temperature cathode spot balance with the discharge current at an intermediate stage of the cathode phase in which the discharge current is increased, the amount of the thermal electrons emitted from the cathode spot at the high temperature comes to exceed the value of discharge current in the initial and final stages of the cathode phase of the electrode at the lighting of the lamp, whereby irregular high frequency oscillations are created to cause noises.

We have made an extensive research of the mechanism of forming the cathode spot and found that the area and temperature of the cathode spot are determined in the following manner: More particularly, as the thermal conductivity of a cathode electrode is not generally sufliciently large with respect to its thermal capacity the cathode spot will be formed in a small area on the surface of the elongated electron emissive body of the cathode structure so that the cathode glow region will also be formed adjacent this cathode spot. When a cathode electrode comprises a coiled elongated electron emissive body the number of ions which are lost in coil turns adjacent the turn on which the cathode spot is formed become large so that the extent of the cathode glow region is limited to a relatively narrow region near the cathode spot. Consequently severe ionizing action of gas or vapor is eifected in this narrow cathode glow region causing ions to concentrate at the cathode spot and thus forming a cathode spot of very small area and of very high temperature. It will be readily understood that the area and the temperature of the cathode spot are determined principally by the magnitude of the discharge current and the extent of the cathode glow region during the temperature limited period.

In conventional discharge lamps, since no effective cooling means is provided the temperature of this cathode spot which has been elevated to an extraordinary value will not be materially decreased at the initial and final stages of the cathode phase in which the discharge current is decreased. Thus, the temperature of the cathode spot is maintained at a sufficient high value enough to cause the quantity of thermal electrons to exceed the discharge current.

Also in the conventional cathode electrode the turns of the cathode coil are densely wound and the mutual radiation of heat of adjacent turns is large, thus preventing the temperature of the cathode spot from falling. Furthermore the above-mentioned concentration of the cathode glow region near the cathode spot will promote concentration of ions and electrons in the accelerating electric field near the cathode spot thus causing irregular high frequency oscillations which result in the creation of noise. We have found that noises are mainly produced by too narrow a concentration of the cathode glow region which produces ions and which forms a cathode spot of extremely high temperature and of extremely small area and that the generation of noise can be effectively precluded by avoiding such an extreme concentration of cathode glow regions.

This invention is based on this conclusion and contemplates to providing cathode electrode or low pressure vapor discharge lamps of extremely low noise by so constructing the cathode structure that the extent of the cathode glow regions is not limited too closely. More particularly in accordance with this invention the cathode electrodes are constructed with reference to the lamp current that the cathode glow region is suitably broadened and the mutual radiation of heat is greatly reduced to prevent the cathode spot from reaching too high a temperature so as to eliminate or greatly reduce generation of noise. These features can be readily attained by proper selection of the construction of the elongated cathode body forming the cathode electrode.

In what follows, an example of the cathode electrode of this invention, criterions of selection of parameters determining the particular construction and some experimental data regarding etfectiveness of the construction are given.

Referring now to the accompanying drawings, in FIGS. la and 1b there is illustrated an example of a cathode electrode embodying this invention particularly suitable for use in household fluorescent lamps of the ratings of from to 40 watts which are now most widely used. The cathode electrode comprises a filament 1 comprising a coiled tungsten wire, said filament being coated with an electron emissive material 2 to have an external diameter A. The coated filament is again wound into a helical coil having an internal diameter B to form an elongated electron emissive body 3. The length of the electron emissive body is many times larger than the diameter A and it is provided at its opposite ends with a pair of lead wires 4 which are sealed at one end of a tubular envelope of the lamp in the conventional manner.

. As has been pointed out above, since this invention contemplates extending the area of the cathode glow region and to prevent excessive concentration of heat at the cathode spot when the lamp is lit, the diameter A of the filament coated with the electron emissive substance 2, the internal diameter B of the coiled elongated body 3 and the spacing D between coil turns which is determined by the coil pitch C are of particular importance.

As can be clearly noted from FIGS. 1a and lb which show the configuration of the elongated body 3 or the relative positions in the space of various surface portions thereof, the internal diameter B, the pitch C and the spacing D between coil turns cooperate to determine the required cathode glow region. Namely, there occurs a cathode spot in a part of the elongated cathode with the resultant generation of a cathode glow around the cathode spot. For proper control of the expansion of the cathode glow, it is required that describing by reference to FIGS. 1a and 1b, the diameter A of the cathode be sufficiently reduced to obtain a thermal conductivity adapted for the heat capacity and that the space around the cathode, or more concretely, the internal diameter B, and spacing D between the coil turns be broadened without obstructing the appropriate expansion of the cathode glow. This can be proved by experiments as described later.

FIG. 2 shows experimental data regarding the relation between noise voltage L and the distance S which defines the extent of cathode glow or the space around the surface of the elongated cathode body, in the case shown in FIGS. la and 1b, -B and/or D which defines the space around the surface of the elongated cathode body 3 which is utilized in a fluorescent lamp containing low pressure mercury vapor and a rare gas. This figure shows that as the spacing is increased to about 1.0 mm., the noises are reduced to about 40 db which is to be compared with conventional lamps wherein noises amount to about 70 to db. Thus this'invention provides fluorescent lamps and the like which produce a low level of interference to radio receivers in the area of low radio field strength.

By the term space used herein is meant a space that can act as cathode glow region and by the term a distance (or dimension) which defines the space around the surface of the elongated cathode body is meant a distance (or dimension) from a portion of the surface of the elongated cathode body to the adjacent turn or other portion, or a length of the space portion measured in normal direction to the surface of the elongated cathode body.

FIG. 3 represents the relation between lamp currents I of various fluorescent lamps and the distance S which defines the space around the surfaces of the elongated electron emissive body when the noise voltage is less than 45 db, as shown in FIG. 2. As shown in FIG. 3 it was found that the distance B or D in FIGS. 1a and 1b is proportional to V], where I represents the lamp current ranged over the magnitude of from 0.2 ampere to 1.0 ampere it being understood that the magnitude of the glow region is determined in accordance with the magnitude of the tube current J.

From the above described analysis of the mechanism of noise generation and from the results of experiments it has been concluded that the objects of this invention can be attained by satisfying the following conditions. Thus, for example, in a cathode electrode in the form of a helically wound coil, the coil internal diameter B and the turn spacing D, both in terms of millimeters, should be larger than 1.1 /J mm. and the diameter of th elongated electron emissive body of the cathode electrode should be smaller than 1.2x].

Dimensions of essential parts of cathode electrode of this invention for fluorescent lamps of various ratings are illustrated in Table I below.

Various fluorescent lamps prepared by sealipg the above described cathode electrodes in glass tubes containing low pressure mercury vapor and rare gases were operated from a source of alternating current through' conventional standard ballasts. The results of measurements made on these lamps show that the level of noise of the respective lamps was about 45 db at a radio broadcasting frequency band of about 1 mc., for example, which coincides with the value shown in FIGS. 2 and 3.

In conventional cathode electrode for use in fluorescent lamps the diameter A of the elongated electron emissive body was too large, or the internal diameter B of the coil was too small or the spacing D between turns was too small when compared with the tube current. Thus all of these dimensions did not satisfy the above described limiting values of this invention. More particularly, the specifications of fluorescent lamps of various wattages which have been manufactured heretofore are as shown in Table II.

TABLE II Internal diameter B Diameter A of the Spacing D between B of the coil (mm.) elongated body (mm.) turns (mm.)

(40 watts .1 =0.435 ampere, standard) 0. 0. 35 0.144 1. 0. 36 0. 29 l. 0. 83 0. 37 0. 0. 34

( watts, J =0.3 ampere, standard) 0. 35 0. 58 0. 35 0. 45 watts, J=0.375 ampere, standard) -It is of course to be understood that the discharge lamps of this invention are not limited to any particular rating described in the Table I and that the configuration of the cathode structure is not limited to the particular construction shown, only required conditions being that the diameter of the elongated electron emissive body should be smaller than approximately 1.2] mm. and that the distance around the surface of the elongated electron emissive body should be larger than about 1.1 /.Tmm., where J denotes the lamp current ranged over the magnitude of from 0.2 ampere to 1.0 ampere. As long as these conditions are satisfied the configuration of the cathode electrode may be of any desired shape. Thus for example the cross section of the helical coil may be an elongated ellipse or rectangular and the spacing 'between turns as well as the internal diameter may be larger than about l.1 /J mm., respectively. Moreover when the coil is wound with a pitch longer than that of commercial helical cathode and with an angle of inclination 0 of the turn of the spiral with respect to the axis of the coil, it is possible to attain the above described objects with different internal diameter of the helix.

- 6 FIGS. 4 to 7 inclusive illustrate modifications of cathode electrode comprising elongated electron emissive bodies 3 of various configurations. Thus in FIG. 4 the elongated body 3 is formed into a conical spiral whereas in FIG. 5, it is formed into a flat spiral with the spacing D between adjacent turns larger than about In FIG. 6 the elongated body 3 is formed into a wavy configuration with the inside diameter B at the crests of the Wave and the spacing D between adjacent linear portions larger than l.1 /J mm., respectively. In the modification shown in FIG. 7, a plurality of elongated bodies 3 are connected in parallel between lead wires 4 with the spacing between adjacent bodies larger than about l.l /Tmm.

The cathode electrode is formed in the form of a coil, an undulate body or a grid by using an elongated electron emissive body or bodies having a diameter A of less than 1.2x] mm. and wherein the internal diameter of the coil and the spacing between coil turns are more than l.1 /7 -mm., where J represents the tube current, ranged over the magnitude of from 0.2 ampere to 1.0 ampere so as to select an adequate extent of the cathode glow region at the timcof discharge whereby to decrease or eliminate noise created in the discharge lamp.

Moreover this construction of the cathode electrode can alleviate concentration of heat at the cathode spot and hence can effectively prevent overheating and degradiation of the electron emissive material. Thus it is not only possible to prolong the useful life of the cathode electrode but also prevent any tendency of increasing noise during the life time. Further by broadening the glow region it is possible to improve the discharge characteristic over the conventional discharge lamps. The discharge can be maintained stably regardless of variations in voltage and ambient temperature, thus increasing the efficiency of the discharge lamp as a source of illumination.

This invention can be readily applied to low pressure vapor discharge lamps of various types and ratings limited without changing their envelopes, electrode mountings method of fabrication and the like.

While the invention has been shown and described with reference to some preferred embodiments of the invention it should be understood that many changes and modifications may be made therein without departing from the true spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A positive column type low pressure vapor discharge lamp comprising a sealed envelope, a filling of inert gas at low pressure and a quantity of mercury contained in said envelope, a pair of electrodes sealed into the ends of said envelope, said electrodes being formed so as to be a self-sustained hot cathode electrode having an electron emissive body and an effective diameter less than 1.2x] mm. and a distance around surfaces thereof longer than 1.1 /7 mm., where J represents the lamp current range of the magnitude of from about 0.2 ampere to about 1.0 ampere.

2. A positive column type low pressure vapor discharge lamp comprising a sealed envelope, a filling of inert gas at low pressure and a quantity of mercury contained in said envelope, a pair of electrodes sealed into the ends of said envelope, said electrodes being formed so as to be a self-sustained hot cathode electrode with an elongated electron emissive body including helically wound heating wire and a coating of electron emissive material applied onto said helically wound heating wire, said elongated electron emissive body being formed into a helical coil to form a coiled coil and having an effective diameter less than 1.2 X] mm. and a coil spacing larger than 1.1 X /7 mm., where J represents the lamp current of the magnitude of from about 0.2 ampere to about 1.0 ampere.

References Cited UNITED STATES PATENTS 1,925,648 9/1933 Spanner et a1. 313-344 X 4/1935 Lowry 313-344 X 10 ROBERT SEGAL, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 

